1. Field of the Invention
The present invention relates to an image forming apparatus provided with a power supply device having a power factor improvement section.
2. Description of the Related Art
Recently, an image forming apparatus is required to enhance the printing speed and reduce time from turning on of a commercial power supply to start of image formation, and an electric power of a heater deployed in a power supply device and a heat fixing device has been increased. In general, an input current supplied from a commercial power supply to the image forming apparatus has an upper limit of something like 15 A (ampere) in Japan, and particularly an image forming apparatus provided with a high-power power supply device and a high-power heater is required to be designed so as not to exceed this upper limit.
In order to satisfy the above requirement, there has been well known an image forming apparatus having a constitution in which electric power is effectively utilized by adding a power factor improvement section to a power supply device. Especially, the power supply device is provided with two DC/CD converters which are a DC/DC convertor supplying electric power mainly to a driving device and a DC/DC convertor supplying electric power mainly to a control device, and in many cases, the power factor improvement section is added only to the former DC/DC converter having a large supply power. As such a power factor improvement section used in a high-power power supply device, the pressure-rising type is often generally used.
However, the power factor improvement section has problems such as heat generation and reduction in efficiency due to switching loss and generation of noise, and it is preferable to operate while stopping switching of the power factor improvement section as much as possible. In order to address those problems, in Japanese Patent No. 3466351, for example, there is disclosed a constitution in which the switching of the power factor improvement section is stopped when an image forming apparatus is in a standby state. Further, in Japanese Patent Application Laid-Open No. 2007-101667, there is disclosed a constitution in which when a value of current flowing to a DC/DC converter which supplies electric power to a driving device and a control device is not more than a predetermined value, the switching of the power factor improvement section is stopped. Furthermore, in Japanese Patent Application Laid-Open No. H04-087565, there is disclosed a constitution in which the power factor improvement section is bypassed by a short circuit.
However, the above patent documents have the following problems. For example, the power factor improvement section disclosed in the Japanese Patent No. 3466351 always performs switching during a printing operation of the image forming apparatus, and there are effects of reduction of heat generation and improvement of the efficiency only when the image forming apparatus is in the standby state. In the first place, in consideration of variation in a commercial power supply voltage and a heater resistance, in order to suppress a value of current supplied from a commercial power supply to not more than a standard of current of 15 A under a condition in which the current value of the image forming apparatus is maximum, the power factor improvement section is provided in the image forming apparatus. Thus, the power factor improvement section is rarely required, and the power factor improvement section is required only during warm up at the time of turning on of the power supply of the image forming apparatus and during a period of time from several seconds to several ten seconds from start of printing at most, and the power factor improvement section may not be required according to the voltage value of the commercial power supply and the heater resistance of a fixing device.
In the constitutions disclosed in the Japanese Patent Application Laid-Opens Nos. 2007-101667 and H04-087565, although the load of the DC/DC converter is significantly different between the printing state and the standby state, a variation in the load of the DC/DC converter is small in the same operating state. Thus, in the printing state in which the load undergoes a transition while remaining large, the switching of the power factor improvement section can be hardly stopped. Accordingly, it is considered that it is less suitable to use the value of the current flowing to the DC/DC converter as a threshold value when whether or not the switching of the power factor improvement section is stopped is judged.